The present invention relates to locating devices for soil displacement hammers.
Soil displacement hammers (commonly referred to as "moles") are increasingly employed for laying underground pipes, cables or conduits underneath a roadway or the like, without the need for excavating a continuous trench and therefore without disturbing the surface or causing disruption to traffic.
Conventional soil displacement hammers are powered by compressed air and operate as follows. A pit or trench is dug at one side of the area to be bored under and the soil displacement hammer is set up and aimed in the direction of the desired cable or pipe run. Compressed air is applied such that the mole bores the cable or pipe hole. Retrieval of the mole may be achieved by digging a corresponding pit at the opposite side of the area being traversed. Soil displacement hammers of this type are described in, for example, Great Britain Specifications Nos. 2,134,152 and 2,147,035, and PCT Specification No. WO87/03924.
Soil displacement hammers are intended to be driven through the ground by repeated pneumatic impact with, typically, 400 to 700 pneumatic impacts per minute in use. They are not intended to be rotatably driven nor to transmit torque, and therefore cannot function as drilling bits which are alternative tools for boring underground tunnels or the like.
While soil displacement hammers operate extremely effectively and will normally cope with small obstructions such as tree roots, stones and the like, larger obstructions can deflect the tool from its predicted course making it difficult to retrieve. One proposal for overcoming the problem has been to attach a radio sonde to the compressed air hose behind the mole by means of waterproof tap or hose clips. However, in practice, such a sonde is not without drawbacks as its shape and the fact that it is asymmetrically located on the hose, give rise to obstruction problems.
The use of a radio transmitter within a housing provided in line with a rotatably driven boring bit has been previously proposed in, for example, U.S. Pat. No. 3,746,106. However, such a boring bit locator cannot easily be adapted for use in a soil displacement hammer, as will now be explained. The only major requirements for the bit locator for a rotatably driven boring bit are that it should be capable of transmitting rotary motion and should permit passage of water to facilitate the cutting action of the boring bit. In contrast, a radio transmitter within a housing provided in line with a soil displacement hammer would be required to permit air to flow therethrough with little or no constriction or disturbance to the air flow, in order that the pneumatic drive transferred to the striker or anvil of the hammer is not adversely affected. It is probably for this reason that the previously proposed solution to the problem of using a radio transmitter with a soil displacement hammer has been to attach the transmitter externally to the compressed air hose.